Low concentration ammonia nanosensor

ABSTRACT

An electrochemical sensor for sensing a gaseous analyte includes a substrate having at least two electrodes disposed thereon, and a carbon nanotube-polyaniline (CNT/PANI) film disposed on the substrate and in contact with at least two electrodes. The CNT/PANI film includes carbon nanotubes coated with a thin layer of polyaniline. The thickness of the polyaniline coating is such that electron transport can occur along and/or between the carbon nanotubes.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of a co-pending applicationhaving U.S. Ser. No. 14/031,322, filed on Sep. 19, 2013, which is acontinuation of PCT Application N. PCT/US2012/055134, filed Sep. 13,2012 and based upon and claiming the benefit of priority from U.S.Provisional Application No. 61/535,645, filed Sep. 16, 2011, the entirecontents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Technical Field

The present invention generally relates to sensors and methods forsensing a gaseous analyte.

Background Information

Gas detection instruments or sensors have a wide range of applications,including industrial health and safety, environmental monitoring, andprocess control. Sensors are used in a variety of fields, includingchemical and petroleum refining, rocket fuel production, fuel cellmanufacturing, semiconductor processing, and biomedical applications.For example, nanoscale sensors can be used to detect certain gaseousanalytes in a sample, such as human exhaled breath. The presence andconcentration of particular analytes may be used to diagnose variousdiseases.

Nanoscale sensors comprise nanomaterials such as carbon nanotubes(CNTs). Nanomaterials can exhibit sensitivity to gases. For example, inview of their unique electrical, thermal, and mechanical properties,CNTs can be used to make gas sensors. Other nanomaterials have alsoshown promise for use as gas sensors.

The sensing mechanism of nanomaterial-based gas sensors depends eitherupon charge transfer between the nanostructure building blocks or, dueto adsorption of charged or polar molecules of the gases on the surfacesof the nanostructure building blocks. An electron donation or withdrawaldue to adsorption of the gas analytes changes the conductivity of thenanomaterials. Nanomaterial-based sensors, therefore, using low-powermicroelectronics technology, convert chemical information into anelectrical signal, leading to the formation of miniaturized sensordevices.

CNTs and other nanostructures, such as nanowires and nanodots, have beendemonstrated as appealing sensing materials for developing advancedchemical gas sensors. Based on the mechanism of charge transfer, gasadsorption (for example, nitrogen dioxide (NO₂), ammonia (NH₂), andoxygen (O₂)) can cause significant electrical transport property changesin the CNTs and nanowires and nanodots, which can be beneficial forsensing applications.

However, gas sensors based on bare nanomaterials, such as pristine CNTs,have limitations, including low sensitivity (due, for example, to lowabsorption capacity), and a lack of selectivity to analytes for whichthey have low adsorption energy or low affinity. This less-than-idealsensitivity and selectivity has limited the use of nanomaterial-basedgas sensors in practical applications. Efforts have been made to improvegas sensitivity and selectivity of CNTs by functionalizing the CNTs withanalyte-specific materials. However, functionalization of the sensorscan require long fabrication time and complicated fabrication steps,which can make the process complex and costly.

Conducting polymers represent one type of sensitive material that hasbeen investigated for CNT functionalization. For example, polyaniline(PANT) has been used as a sensing material for a variety of toxic gasessuch as carbon monoxide, nitrogen dioxide, and ammonia. PANI exhibitsp-type semiconductor characteristics, so electron-supplying gases suchas ammonia reduce the charge-carrier concentration and decrease theconductivity of the polymer. However, CNT functionalization can involvecomplex fabrication processes, and resulting functionalized CNTs canlack the desired properties for sensing applications.

A need exists for improved CNT gas sensors and methods that do notrequire a complex fabrication process or high operating temperatures,and that have the desired properties for sensing applications,including, for example, low resistivity and high resolution,specifically at low analyte concentrations.

SUMMARY OF THE INVENTION

Briefly, the present invention satisfies the need for gas sensors andmethods that do not require a complex fabrication process or highoperating temperatures, and that have desired properties for sensingapplications. The present invention may address one or more of theproblems and deficiencies of the prior art discussed above. However, itis contemplated that the invention may prove useful in addressing otherproblems and deficiencies in a number of technical areas. Therefore, theclaimed invention should not necessarily be construed as limited toaddressing any of the particular problems or deficiencies discussedherein.

Certain embodiments of the presently-disclosed gas sensors and methodsfor producing gas sensors have several features, no single one of whichis solely responsible for their desirable attributes. Without limitingthe scope of these gas sensors and methods as defined by the claims thatfollow, their more prominent features will now be discussed briefly.After considering this discussion, and particularly after reading thesection of this specification entitled “Detailed Description of theInvention,” one will understand how the features of the variousembodiments disclosed herein provide a number of advantages over thecurrent state of the art. These advantages may include, withoutlimitation, simplified and efficient methods of fabricatingelectrochemical gas sensors having good sensing properties, such as highsensitivity, low detection limits, fast response and recovery times,good reproducibility, good selectivity in the detection of gases, andlong term stability.

The present invention provides, in a first aspect, an electrochemicalsensor for sensing a gaseous analyte. The sensor includes a substratewhich has two or more electrodes disposed thereon, and a carbonnanotube-polyaniline film disposed on the substrate and in contact withat least two electrodes. The carbon nanotube-polyaniline film includescarbon nanotubes coated with a thin layer of polyaniline having athickness such that electron transport can occur along and/or betweenthe carbon nanotubes.

The present invention provides, in a second aspect, a method of makingan electrochemical sensor for sensing a gaseous analyte. The methodincludes preparing a solution of polyaniline and camphorsulfonic acid inchloroform, preparing a solution of octadecylamine functionalized carbonnanotubes in chloroform, mixing the two solutions, and depositing themixed solution on a substrate which has at least two electrodes disposedover it, to form a carbon nanotube/polyaniline film on the substrate,such that the film is in contact with at least two electrodes. The filmcomprises carbon nanotubes coated with a thin layer of polyaniline ofthickness such that electron transport can occur along and/or betweenthe carbon nanotubes. The film can be formed by spin-coating the mixedCNT/PANI solution.

These and other features and advantages of this invention will becomeapparent from the following detailed description of the various aspectsof the invention taken in conjunction with the appended claims and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mechanism for chemical sensing using polyaniline.

FIG. 2 is a top perspective view of one embodiment of an electrochemicalsensor according to the present invention.

FIG. 3 is a graph showing the kinetics of the resistance of a PANIsensor.

FIG. 4 is a graph showing the concentration dependence of the resistanceof a PANI sensor.

FIG. 5 is a graph showing the kinetics of the resistance of a CNT/PANIsensor according to the present invention.

FIG. 6 is a graph showing the concentration dependence of the resistanceof a CNT/PANI sensor according to the present invention.

FIG. 7 depicts an example of an experimental setup for the quantitativemeasurements of gas sensors of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to electrochemical sensorsfor sensing a gaseous analyte, and to methods of making electrochemicalsensor for sensing a gaseous analyte.

Although this invention is susceptible to embodiment in many differentforms, certain embodiments of the invention are shown. It should beunderstood, however, that the present disclosure is to be considered asan exemplification of the principles of this invention and is notintended to limit the invention to the embodiments illustrated.

The presently-disclosed electrochemical sensors, in one aspect, providea device for detecting or sensing a particular gas or gases (e.g.,hydrogen, hydrogen sulfide, ammonia, nitrogen dioxide, water vapor,oxygen, or methane) in a gaseous environment. In various embodiments,the sensors comprise carbon nanostructures, which are coated with amaterial which is used to functionalize the nanostructure.

In some embodiments, the carbon nanostructures are carbon nanowiresand/or carbon nanotubes (CNTs), for example, single-walled nanotubes(SWNTs) or multi-walled nanotubes (MWNTs), although this list isnon-limiting and other carbon nanostructures may also be used.

In some embodiments, the material used to functionalize thenanostructures is chosen based on the gaseous analyte(s) to be detected.For example, in some embodiments, the material comprises palladium (usedfor detecting, e.g., hydrogen), gold (used for detecting, e.g., hydrogensulfide or mercury vapor), or a metal oxide (used for detecting, e.g.,methane and/or ammonia). In certain embodiments, the material used tofunctionalize the nanostructures, such as CNTs, is a conductive polymer.For example, in some embodiments, the material comprises polyaniline(PANI) (used for detecting, e.g., ammonia, water vapor, and/or nitrogendioxide).

In certain embodiments, the gas sensors and methods of the presentinvention use CNTs functionalized with polyaniline (PANI) doped withcamphor-sulfonic acid (CSA). Such sensors may be used for detectingammonia and/or nitrogen dioxide. PANI(CSA) is a typical sulfonatedconducting polymer having tunable electronic properties coupled withgood environmental and thermal stability. Gas sensors comprising CNTs(e.g., MWNTs) functionalized with PANI(CSA) exhibit humidityindependence. Thus, these sensors can be used in a wide range ofenvironments without the need to compensate for relative humidity,making the process of using the sensors simpler and less expensive.

The sensing mechanism of the sensors according to certain embodiments ofthe present invention is based on a charge interaction between themolecules of a specified gaseous analyte and the functionalized carbonnanostructures. Specifically, the electrical properties of theelectrochemical sensor, particularly its resistance, change from apredetermined baseline value upon exposure to a gaseous environmentcontaining the specified gaseous analyte as a result of thefunctionalized nanostructures accepting electrons from, or donatingelectrons to the molecules of a specific gaseous analyte to which thesensor is exposed, and to which the materials of the sensor areresponsive.

In some embodiments, the change in a measured electrical property (e.g.,resistance) is measured or quantified, directly or indirectly. Forexample, a change in resistance may manifest itself as a change incurrent flowing through a circuit including the sensor or a change inthe voltage drop across the sensor. The change in current or voltage isthen measured and calibrated to a quantifiable gas concentration value.The magnitude of the change in the measured parameter establishes theconcentration of the specified gas present in the system when comparedto a baseline or calibration value of the measured parameter. Thesetypes of sensors, also referred to as chemiresistive sensors, havedesirable properties, such as high sensitivity, low detection limits,fast response and recovery times, good reproducibility, good selectivityin the detection of gases, and/or long term stability, such as within arange of temperatures.

In certain embodiments of the present invention, to assess thesensitivity of the sensors, measurements of the electrical signals areprocessed using direct current (DC) technique. The signals are digitizedusing standard acquisition boards, and the experimental information isprocessed using computers.

In certain embodiments of the present invention, the material used tofunctionalize the nanostructures is polyaniline. FIG. 1 shows amechanism for chemical sensing using polyaniline. The nitrogen atomsalong the PANI backbone make the material particularly sensitive toammonia. Upon exposure to ammonia (NH₃), the emeraldine salt form ofpolyaniline is deprotonated to yield the less conducting emeraldine baseform, and the ammonia forms more energetically favorable NH₄ ⁺ ions. Thechange in resistivity from highly conducting to practically insulatingis used to gauge the extent of chemical reactivity with the nitrogenbonding sites along the PANT backbone. Exposure of the PANI emeraldinesalt to ammonia vapors produces this reversible de-doping effect,allowing sensors to be reused.

FIG. 2 is a top perspective view of one embodiment of an electrochemicalsensor 10 according to the present invention. In the depictedembodiment, the gas sensor comprises a substrate 20, which is held bysubstrate holder 24. While the substrate 20 shown in the depictedembodiment is a 20×10 mm substrate, persons having ordinary skill in theart will readily understand that substrates of various alternative sizesare conducive to the instant invention. In embodiments of the presentinvention, the substrate 20 is a nonconductive or insulating material,such as glass. The substrate 20 has at least two conductive electrodes,formed or disposed thereon. In some embodiments, the electrodes extendfrom the contact pads 22 and lie in the active area 30 on substrate 20.Persons having ordinary skill in the art will understand that the sizeand shape of the active area 30 can vary depending on the number anddimension of the electrodes. For example, although not limiting, incertain embodiments the active area 30 tray be hundreds of microns orseveral millimeters in width and length, for example, from 1×1 mm to 8×8mm, such as 4×4 mm.

In some embodiments, the electrodes may be formed on substrate 20 in asubstantially parallel array, using any suitable method known in theart, for example, conventional photolithography, screen printing,electrodeposition, or standard sintering techniques. In certainembodiments, the electrodes may be formed of metals that do not oxidizeeasily, such as, for example, nickel, gold, platinum, palladium or othermetals known in the art to be suitable for this application. Personshaving ordinary skill in the art will understand that the electrodes maybe of various widths and lengths. For example, although not limiting, incertain embodiments the electrodes may be in the range of 1 μm to 50 μmin width. The electrodes are separated by one or more gaps having awidth which can vary. Although not limiting, in certain embodiments, thewidth of the gap(s) is in the range of several hundred nanometers tomore than 100 μm. In one embodiment, the width of the gap between twoelectrodes is between 1 μm and 10 μm, such as, for example, 5 μm. Whilethe substrate 20 comprises at least two electrodes, various embodimentsof the present invention include more than two electrodes, such asthree, four, five, six, or more electrodes. For example, although notpictures, in the depicted embodiment, the substrate 20 comprises fourparallel planar platinum electrodes with 5 μm separation.

In certain embodiments of the present invention, a carbonnanotube-polyaniline (CNT/PANI) film (not visible in FIG. 2) is disposedon the substrate 20 such that it is in contact with at least twoelectrodes. The film may be disposed on the substrate 20 by anyconventional methods known in the art, for example, by using the spincasting technique. In embodiments of the present invention using thespin casting technique, the resulting film thickness can be varied bychanging the spin speed or the concentration of CNT/PANI in the solutionto be spin-cast. PANI/CNT films are spin cast from chloroform at variousspeeds, for example, from 1000-3000 RPM at the system of electrodes.

In some embodiments, the CNT/PANI film comprises carbon nanotubes e.g.,MWNTs or SWNTs) coated with a layer of polyaniline of thickness suchthat electron transport can occur along and/or between the coated carbonnanotubes.

In certain embodiments of the present invention, the average thicknessof the polyaniline coating on the CNTs is less than the average diameterof the carbon nanotubes. In some embodiments, the polyaniline coatinghas an average thickness of less than 10 nm, or less than 9 nm, or lessthan 8 nm, or less than 7 nm, or less than 6 nm, or less than 5 nm, orless than 4 nm, or less than 3 nm, or less than 2 nm. In certainembodiments, the layer of polyaniline is transparent to electrontunneling, e.g., the energy barrier is low enough, and the thickness ofthe polyaniline layer separating two quantum wells (carbon nanotubes) isthin enough such that the probability of tunneling of a charge carrier(electron) in nonzero. This means, the PANI forms very thin gaps betweennanotubes that are transparent to electron tunneling. When the layer ofpolyaniline is transparent to electron tunneling from CNT tube to tube,the conducting mechanism of the CNT/PANI film is due to electrontransport along the tubes and electron tunneling through PANE gaps.“Tunneling thin” layers are discussed in Ostroumova et al.,Semiconductors 33 (9), 1027-1029 September 1999, which is herebyincorporated by reference with respect to relative portions related tothe subject matter of the present invention and all of its embodiments.

In certain embodiments of the present invention, the average diameter ofthe polyaniline-coated multiwall carbon nanotubes is greater than orequal to 10 nm and less than 50 nm. For example, in certain embodimentsthe average diameter of the polyaniline-coated carbon nanotubes is 10-45nm, 10-30 nm, or 15-40 nm, or 20-40 nm.

In certain embodiments of the present invention, the CNT/PANI film hasan average thickness between 40 and 400 nm. For example, in certainembodiments, the CNT/PANI film has an average thickness of 40-300 nm, or40-250 nm, or 40-200 nm, or 40-100 nm, or 50-300 nm, or 50-250 nm, or50-200 nm, or 50-100 nm, or 100-200 nm, or any intermediate rangefalling within one of the defined ranges (e.g., of thickness in rimhaving a range of any integer greater than 40 to any integer less than300).

In some embodiments, gas sensors of the present invention are capable ofsensing less than 400 ppm of a gaseous analyte, for example, less than200 ppm or less than 100 ppm or less than 50 ppm or less 40 ppm or lessthan 30 ppm or less than 20 ppm or than 10 ppm or less than 1 ppm. Insome embodiments, the gas sensors of the present invention are capableof sensing less than 500 ppb of a gaseous analyte, for example, lessthan 400 ppb or less than 300 ppb or less than 200 ppb or less than 175ppb or less than 150 ppb or less than 125 ppb or less than 100 ppb orless than 75 ppb or less than 50 ppb or less than 25 ppb. In someembodiments, the gas sensors of the present invention are capable ofsensing concentrations greater than or equal to 1 ppb of a gaseousanalyte. According to certain embodiments, gas sensors of the presentinvention exhibit reversible sensor response toward gaseous analytes inthe foregoing concentration ranges.

FIG. 7 depicts an example of an experimental setup for the quantitativemeasurements of gas sensors of the present invention. In the depictedsetup, the gas sensor 1 is located at the testing gas flow bench 2.Digital flow controllers 3 and 3′ are connected to the pressureregulators 4 and 4′, which are connected to gas cylinders 5 and 5′. Thegas cylinders 5 and 5′ may comprise, for example, N₂ as a carrier gas.The gases for testing can be different and/or diluted. Currentpre-amplifier 6 and voltage preamplifier 7 are configured with AIDconverter 8 and computer 9. The assembled configuration allowscontrollable flow rate of the testing gases from, for example, 1 to 100sccm in the controllable flow rates from 10 to 1000 sccm.

The present invention also provides methods of detecting the presence ofspecific gaseous analytes in a gaseous environment (e.g., air), by usingthe gas sensors of the present invention. In certain embodiments, themethods include determining a baseline value of an electrical parameter(such as resistance) of the sensor, exposing the sensor to a gaseousenvironment that may include the specific gaseous analyte to bedetected, and measuring any change in the electrical parameter value ofthe sensor after exposure to the gaseous environment. In one specificembodiment, the method is for detecting ammonia.

The present invention also provides methods of making an electrochemicalsensor for sensing a gaseous analyte. In certain embodiments, the methodcomprises: (a) preparing a solution of polyaniline (PANI) andcamphorsulfonic acid (CSA) in chloroform; (b) preparing a solution ofoctadecylamine (ODA) functionalized carbon nanotubes (CNTs, for example,MWNTs) in chloroform; (c) mixing the PANI/CSA solution and theODA-functionalized CNT solution to make a CNT/PANI solution; and (d)depositing a CNT/PANI film on a substrate having at least two electrodesdisposed thereon, such that the film is in contact with at least twoelectrodes. In certain embodiments, the CNT/PANI film comprises carbonnanotubes coated with a layer of polyaniline of thickness such thatelectron transport can occur along and/or between the carbon nanotubes,and said film is formed by any acceptable technique known in the art,for example, by spin-coating the CNT/PANI solution.

In certain embodiments of methods according to the present invention,the concentration of PANI in the PANI/CSA solution is equal to theconcentration of CNTs in the ODA-functionalized CNT solution. In someembodiments, the CNT/PANI solution is a 50:50 mixture of PANI/CSAsolution and ODA-functionalized CNT solution.

In certain embodiments of methods of making an electrochemical sensorfor sensing a gaseous analyte according to the present invention, thegaseous analyte is ammonia. In some embodiments, the methods of thepresent invention produce sensors capable of sensing at least 1 ppb ofammonia.

In certain embodiments, the present invention provides sensors andmethods having advantageous sensitivity, response and refraction times,linearity, bias stability, thermal stability, and/or regeneration rateof the gas sensors. For example, in some embodiments, the presentinvention provides sensors and methods having sensitivity to analytes atconcentrations of 1 ppb. In some embodiments, the present inventionprovides sensors having a linear response to low analyte concentrations(e.g., to concentrations of, for example, less than 200 ppb, or lessthan 180 ppb, or less than 150 ppb, or less 120 ppb, or less than 100ppb).

An embodiment of the above-described gas sensor was prepared for use ingas detection studies.

Samples Preparation.

Emeraldine base polyaniline (PANI-EB) with a molecular weight of 10andcamphor-10 sulphonic acid B were purchased from Aldrich and used asreceived. PANI-EB and CSA were dissolved in chloroform at a molar ratioof 1:0.5 to completely protonate the PANI backbone to produce theemeraldine salt form (PANI-CSA). Solutions were stirred for 3 days andsonicated for 15 minutes prior to preparing PANI thin films. PANI/CNTfilms were made from PANI and CNT solutions in chloroform of equalconcentrations (1 mg/ml). In order to reach this high solubility of CNTchloroform CNTs were functionalized with ODA. ODA functionalized CNTswere processed in the Lab, but can be purchased, for example, from M KImpex Corp. Division: MKnano.

PANI and CNT solutions are of equal concentration, a 50:50 blendsolution was prepared by adding equal amounts. For our sensors we used50:50 solution to compare with 100% PANI-doped by CSA.

Thin films were prepared using the spin casting technique on clean glasssubstrates with pre-patterned platinum electrodes. Glass substrates werecleaned via sonication in acetone followed by rinsing in de-ionizedwater. Polymer films were spin cast at 1000 RPM for 45 seconds toproduce films 100-200 nm thick, which covered the substrate. A sectionof the thin films were removed from the Pt finger electrodes to ensuredirect electrical contact during measurements by using a combination ofO₂/Ar plasma in a March Plasma RIE. FIG. 2 is a top perspective view ofthe sensor along with the sensor holder. The polymer (e.g., CNT/PANI)film thickness can be varied by changing the spin speed or solutionconcentration.

Sensitivity of the Gas Sensor.

The sensitivity of the sensor to ammonia in the ppb range ofconcentrations was measured using a set-up similar to that depicted inFIG. 7. The diluted in N₂ to 100 ppm concentration ammonia gas wasinjected with flow rates under 1 sccm into the stream of extra dry N₂gas, flowing with the fixed rate of 1000 sccm in the quartz chamber withthe sensor inside.

The potential difference of 0.1 Volts was applied to the sensor in orderto detect a current variation under the influence of the ammonia gas.

The current was converted to a voltage signal by a precisioncurrent/voltage preamplifier and digitized by a Ni A/D converter.

A 50:50 blend of (1) PANI-doped by CSA in chloroform and (2) ODAfunctionalized CNT in chloroform, the PANT and CNT solutions each havingequal concentrations, was mixed to form a solution of PANI-CSA:ODA-CNT(the “CNT/PANI solution”) in chloroform. The resulting CNT/PANI solutionwas tested and compared with 100% PANI-doped by CSA.

PANI and PANI/CNT films were spin cast from chloroform at speeds varyingfrom 1000-3000 RPM at the system of electrodes.

The PANI-CSA thin film samples were characterized by measuring currentflow as a function of exposure to ammonia vapors in nitrogen atmosphereat room temperature at fixed applied voltage. The duration of sensorexposure to ammonia and the concentration of ammonia gas were varied.

The resistance of the sensors was measured dependently uponconcentration of ammonia (in the range of 100 ppb-10 ppm).

A graph showing the kinetics of the resistance of a PANI sensor ispresented in FIG. 3, and the concentration dependence of the resistanceis presented in FIG. 4. A graph showing the kinetics of the resistanceof the PANI-CNT (50-50%) sensor is presented in FIG. 5, and theconcentration dependence of the resistance is presented in FIG. 6. Theplots of FIGS. 2-6 demonstrate that the CNT-PANI nano-composite sensorspossess lower resistivity and higher sensitivity than the PANI sensorwithout CNTs. This is evidenced by, for example, a significantdifference (in this case, one order of magnitude) in the resistance ofthe pure PANI sensors (˜3.7×10⁵ Ohm; see baseline, FIG. 4) and theCNT/PANI sensor made according to the instant invention (˜3.6×10⁴ Ohm;see baseline, FIG. 5). In certain embodiments, this high sensitivity isattributable to the thin tunneling PANI layer, and to electron transportboth along the CNTs and through the PANI gaps between nanotubes. Asshown in FIG. 5, in certain embodiments, sensors according to thepresent show reversible sensor response to even low concentrations ofgaseous analyte. FIG. 5 reflects a return to the baseline even whentested at the lowest analyte concentration, 100 ppb. The figure suggeststhat the sensors are reversibly sensitive to even lower analyteconcentrations.

Based on the lowest measured signal of 100 ppb in FIG. 5, it isestimated that sensors according to the present invention are able todetect as low as 1 ppb ammonia in gaseous environments (e.g., dry N₂).

While several aspects and embodiments of the present invention have beendescribed and depicted herein, alternative aspects and embodiments maybe affected by those skilled in the art to accomplish the sameobjectives. Accordingly, this disclosure is intended to cover all suchfurther and alternative aspects and embodiments as fall within the truespirit and scope of the invention.

1.-22. (canceled)
 23. An electrochemical sensor for sensing a gaseousanalyte comprising: a substrate comprising at least two electrodesdisposed thereon; and a carbon nanotube-polyaniline (CNT/PANI) filmdisposed on said substrate and in contact with at least two electrodes,wherein said CNT/PANI film comprises octadecylamine (ODA) functionalizedcarbon nanotubes.
 24. The electrochemical sensor of claim 23, whereinsaid CNT/PANI film further comprises ODA functionalized carbon nanotubescoated with a layer of polyaniline of thickness such that electrontransport can occur along and/or between the ODA functionalized carbonnanotubes.
 25. The electrochemical sensor of claim 23, wherein said ODAfunctionalized carbon nanotubes are multi-walled carbon nanotubes. 26.The electrochemical sensor of claim 25, wherein said multi-walled carbonnanotubes have an average diameter that is greater than or equal to 10nm and less than 50 nm.
 27. The electrochemical sensor of claim 23,wherein said electrodes are platinum electrodes.
 28. The electrochemicalsensor of claim 23, wherein the CNT/PANI film has an average thicknessof 50-100 nm.
 29. The electrochemical sensor of claim 23, wherein saidcarbon nanotube-polyaniline (CNT/PANI) film comprises ODA functionalizedmulti-walled carbon nanotubes coated with polyanailine doped withcamphor-sulfuric acid (PANI-CSA).
 30. The electrochemical sensor ofclaim 29, wherein the PANI-CSA coating has an average thickness of lessthan 10 nm.
 31. The electrochemical sensor of claim 23, wherein thegaseous analyte is ammonia.
 32. The electrochemical sensor of claim 31,wherein said electrochemical sensor is capable of sensing at least 1 ppbof ammonia.
 33. The electrochemical sensor of claim 23, wherein thesubstrate comprises four to six electrodes disposed thereon.
 34. Theelectrochemical sensor of claim 33, wherein said substrate comprisesfour electrodes disposed thereon.
 35. The electrochemical sensor ofclaim 33, wherein said substrate comprises five electrodes disposedthereon.
 36. The electrochemical sensor of claim 33, wherein saidsubstrate comprises 6 electrodes disposed thereon.
 37. A method fordetecting ammonia in a gaseous sample, comprising: an electrochemicalsensor of claim 23 to a gaseous sample, wherein said electrochemicalsensor changes resistivity in response to an amount of ammonia presentin said gaseous sample; detecting said change in resistivity in responseto ammonia in said gaseous sample by detecting a concentration ofammonia less than 200 ppb; and measuring the concentration of ammoniabased on said change in resistivity of said electrochemical sensor. 38.The method of claim 37, wherein said exposing said electrochemicalsensor to a gaseous sample comprises: obtaining a baseline ammoniaconcentration reading from said gaseous sample; and measuring the changein current or voltage across said electrochemical sensor, wherein themagnitude of the change in current or voltage establishes theconcentration of ammonia present.
 39. The method of claim 37, whereinsaid gaseous sample is a breath sample.
 40. The method of claim 37,wherein said concentration of ammonia in said sample is less than 100ppb.
 41. The method of claim 37, wherein said concentration of ammoniain said sample is between 1 ppb and 200 ppb.
 42. The method of claim 41,wherein said concentration of ammonia in said sample is about 1 ppb.